Heat recovery process



May 3, 1966 J. R. BUSS ETAL HEAT RECOVERY PROCESS Filed Jan. 2, 1964 FIG.

| -90 VAVAV/JVAVA A VA VA VA VA "3 ii i i FIG.

INVENTORS JOHN R. BUSS MALCOLM MCEWEN BY M @W FIG.|

ATTORNEY United States Patent 3,249,152 HEAT RECOVERY PROCESS John R. Buss and Malcolm McEwen, St. Louis, Mo., assignors to Monsanto Company, a corporation of Delaware Filed Jan. 2, 1964, Ser. No. 335,035 4 Claims. (Cl. 165-1) The present invention relates to an improvement in the recovery of waste heatfrom high temperature exhaust gas streams such as boiler stack gases. It has long been recognized in the operation of boilers both of the production of process steam and in power production, as well as in the operation of internal combustion engines that a large proportion, e.g., as much as 25% of the heat of combustion cannot economically be recovered since such heat is lost as sensible and latent heat of the gaseous products of combustion which leave the boiler through the stack or chimney.

It is an object of the invention to improve the thermal efficiency of plant systems such as boilers and diesel engines by recovering a larger portion of the heat which would otherwise be lost as sensible and latent heat leaving the boiler stack or engine exhaust. It is a further object of the invention to employ a specific group of chlorinated polyphenyl compounds as heat recovery media in carrying out the object of the invention;

The present invention departs from the prior art in employing a specific organic chemical composition for direct contact with the hot gas stream for the recovery of heat. The organic compositions which are employed in the practice of the present invention are a specific group of halogenated biphenyl compounds having from 40 to 60% by weight of chlorine, a preferred range being 45% to 55% chlorine.

The above working materials employed in practicing the present invention are characterized by chemical inertness, fire resistance and low vapor pressure. The present class of compounds have been found to be far superior to hydrocarbons such as oils because of the oxidative stability characteristic of the molecular structure. Furthermore, the present groups of materials has a density greater than that of water, so that, as described herein, a phase separation from water is possible. While not critical, it has been found desirable to employ materials having a vapor pressure less than about 0.1 mm. at 100 C.

Essentially the present invention comprises the recovery of heat from hot gases by heat exchange with the present group of working compounds. Because of the stability of these compounds they can be directly contacted with the hot gases such as by spraying droplets of the compounds down into a tower in which the hot gases are flowing upward. Thus, gases can be used at 300 F. to 1000 F. or more as obtained from a boiler, gas turbine, or from a diesel engine. This direct contact step increases the temperature and heat content of the synthetic organic compound, and can also result in some condensation of water from the water vapor content of combustion gases, thereby also adding further heat to the working medium.

After the organic compound has been increased in heat content it is used for heat exchange. In one embodiment of the invention, the working medium is used in an indirect heat exchanger, e.g., such as in a shell and tube heat exchanger to heat feedwater or a process stream. Another embodiment is to use direct heat exchange such as in mixing organic liquid and a heat acceptor liquid such as water or other inert liquid in a vessel. For example the organic medium is passed downward through a large tank while feedwater is introduced at the lower portion. The use of distribution nozzles and a settling section permits easy separation because of the high density of the organic working liquid. After the heat 3,249,152 Patented May 3, 1966 ICE content of the organic compound has been recovered, this component is recycled to the heat pickup step. The feed water is withdrawn and used in a boiler or for process use, etc. Instead of using a single contacting stage, a number of stages may also be employed in order to improve the efiiciency.

The chlorinated materials of the present invention are particularly useful in the present process since, in the specific range of chlorine content set forth herein, these working materials are completely fire resistant so that they may be employed in installations where fire hazards may be present. Another advantage which has been found to be characteristic of the present group of chlorinated ring compounds is their high density, e.g., from 1.3 to 1.6 absolute density. This is important in the separation of Water. Accordingly, the present materials are readily separated as a heavy liquid phase from the aqueous phase.

The above halogenated materials are also characterized by exceedingly good high temperature stability. For example it has been found that all of the present class of chlorinated polyphenyl compounds when subjected to a temperature of 500 F. exhibit less decomposition than 0.0002 wt. percent/hour.

The properties of these materials, i.e., their chemical composition, consisting only of carbon, hydrogen, and halogen and high thermal stability make possible the operation of recovery units with these materials at temperatures as high as 425 C. or higher for extended periods of time.

The chlorinated biphenyl containing from about 40% to about 60% by weight of combined chlorine are generally useful as the preferred operating fluids of this invention, however, chlorinated biphenyl containing from about to by weight of combined chlorine is preferred as providing fluids having superior viscosity characteristics. Chlorinated biphenyl within this preferred range is commercially available as products containing about 42%, 48%, 54% or (wt) of combined chlorine corresponding approximately to tri-, tetra-, pentaand hexachlorobiphenyl, respectively. The expressions chlorinated biphenyl containing 40% to 60% of combined chlorine, and chlorinated biphenyl containing 45% to 55% of combined chlorine, are used herein as not only including these direct chlorinated products, but also as blends of one or more of the chlorinated biphenyls whereby the total chlorine content is broadly Within the range of 40% to 60%, preferably within the range of 45% to 55% by weight. For example, chlorinated biphenyl containing a total of 45 by weight of combined chlorine can be effectively prepared, for the purpose of this invention, by blending together 50 parts by weight of chlorinated biphenyl containing 42% by weight of combined chlorine and 50 parts by weight of chlorinated biphenyl containing 48% by weight of combined chlorine. In a similar manner, to further illustrate, chlorinated biphenyl containing a totalof 58% by weight of combined chlorine, for purposes of this invention, may be effectively prepared by blending together 25 parts by weight of chlorinated biphenyl containing 52% by weight of combined chlorine and parts by weight of chlorinated biphenyl containing 60% by weight of combined chlorine. Therefore, for purposes of this invention, the chlorinated biphenyl containing generally 40% to about 60%, or preferably 45 to 55% by weight of combined chlorine may be obtained either by direct chlorination of the biphenyl to obtain the desired combined chlorine content, or a satisfactory material may be obtained by blending together two or more chlorinated biphenyls to obtain a resulting blend of chlorinated biphenyl containing an effective quantity of combined chlorine within the ranges designated above.

Chlorinated biphenyl, as described above, comprises the major proportion of the chlorinated aromatic hydrocarbon content of the operating fluids of this invention, and it is generally preferred that such chlorinated biphenyl constitute at least 75% and even more of the entire chlorinated aromatic hydrocarbon content of the fluids of this invention. On the other hand, while the chlorinated aromatic hydrocarbon content is made up of at least 50% by weight of chlorinated biphenyl, the remaining 50% of chlorinated aromatic hydrocarbon content can be other chlorinated aromatic hydrocarbons such as trichlorobenzene and partially chlorinated terphenyl. In particular, the use of trichlorobenzene in an amount so as to equal up to about to of the chlorinated aromatic hydrocarbon content of the fluid is quite desirable where a fluid is to be used under conditions of low temperature, since the pour point temperature and viscosity at low temperatures of the fluid can be significantly reduced by such use .of trichlorobenzene.

The above described synthetic organic compounds are essential components of the working fluid. Adjunct materials which can be added to reduce volatility, control viscosity, vapor pressure, as well as specific problems arising in various installations with respect to foaming or emulsion formation include silicones, aryl silanes and various aryl phosphorus compounds such as triphenyl phosphate.

The direct-contacting heat transfer media employed in the practice of the present invention are of special utility because of their high thermal stability so that gases at unusually high temperatures can readily be employed for heat recovery. Furthermore, the presence of sulfur dioxide and sulfur trioxide in the gases undergoing treatment does not cause deterioration or breakdown of the working fluid. In addition, the cycling of the medium through heat exchangers, or in direct contact with other liquids such as water for heat exchange does not cause significant decomposition of the present organic working media. Thus, the difiiculties of prior art systems in attempting to operate at high temperatures are to a considerable degree overcome by the present process.

There are specific situations in the recovery of heat from large volumes of gases, for example in working with the flue gases from large power plants, where a multi-stage heat recovery system is desirable.- In such situations, a two stage system or higher number of units maybe employed with the present synthetic organic compound being employed in one or more of such stages. Another embodiment of-the invention is to employ the present high thermal stability material in the primarystage in which gases of highest temperature are directly contacted, and to use another inert fluid such as water as the liquid medium in the second or additional stages. Examples of these modifications are shown in the drawings and examples forming a part of the present patent application.

In the drawings of the present application, FIGURE.

1 shows a single stage heat recovery system using the organic compounds described above as the heat recovery medium. FIGURE 2 illustrates a heat recovery system employing two working fluids and with indirect heat exchange in the heat recovery portion of the cycle. FIG- URE 3 shows another two stage heat recovery system employing direct contact of two working fluids for heat recovery.

In FIGURE 1 a directcontact heat recovery system is energy, it is recycled by line- 10 to contactor 1. The

heat acceptor fluid such as boiler feedwater enters heat exchanger by line 21 and leaves by line 22.

As an example of the operation of the present heat exchange system, a boiler flue. gas enters tower 1 above at the rate of 275,000 lb. per hour at a temperature of 380 F. The working medium, chlorinated biphenyl having 54 Wt. percent chlorine is passed into the tower at the rate of 300,000 lbs. per hour with an incoming temperature of F. The gas stream leaving the tower is at a temperature of F. while the medium leaving at the bottom of the countercurrent contacting tower is at a temperature of 330 F. The heat acceptor fluid used in the indirect heat exchanger at a stream flow of 222,000 lbs. per hour of feedwater which enters at 50 F. and leaves the heat exchanger at 123 F.

Analysis of the recovery in the above system shows an improvement corresponding to an increase of 7% in the net steam output from the boiler plant for the same amount of fuel consumed. For the same net boiler plant steam output as the conventional system a reduction in fuel consumption of 6.5% is achieved.

In FIGURE 2 a two liquid heat exchange system is employed. In FIGURE 2 the boiler flue gases 31 are introduced through duct 3-2 into the high temperature cont-acting tower 33. The gases. pass upward through this tower through duct 34 containing a de-entrainer 37, and then enter the low temperature tower 35. .In tower 35 the gases again pass upward and leave the tower through a de-entrainer 36. In the dual fluid heat recovery system of FIGURE 2, the heat recovery medium in the low temperature tower is the incoming water stream entering through line 40 and being dispersed through nozzles 41 to pass countercurrent through flue gases to the bottom of tower 35 and to be withdrawn through-line 42 whichlis also provided with an organic-phase separator 44. The high temperature loop employs a chlorinated polyphenyl compound, a suitable type being a chlorinated biphenyl having 48% by weight of chlorine. The chlorinated biphenyl enters tower 33 through line 50, and is dispersed as droplets by nozzles 51. The droplets pass tcountercurrent to the flue gas and are collected at the bottom of tower 33 to leave by way of line 52. The chlorinated biphenyl stream then passes through a filter '53. The stream of chlorinated biphenyl goes through line 54 to enter heatexchanger 55 and is thereafter recycled through line 50. The recovery of the higher temperature heat from this loop of the recovery system is carried out in heat exchanger 55 which receives the partially heated water from line 42 and discharges the further heated water through line 56. The heated water is thus brought to a higher temperature by the recovery of some of the heat content of the boiler flue gases- In the application of the present heat recovery fluid chlorinated biphenyl having 48 wt. percent chlorine, the temperature of the gas stream entering the heat recovery unit is 381 F. with the gas flovv amounting to 275,000 lbs. per hour. The gas stream leaving the first tower is at a temperature of F. while the gas temperature at the exit of the second tower is 100 F. The high temperature tower uses the synthetic organic medium described above, entering at the rate of 610,000 lbs. per hour at a temperature of 145 F., and leaving the first tower at 245 F.' In'the second tower the heat recovery medium is water entering at the rate of 207,000 lbs. at a temperature of 50 F. The water leaving the second tower then goes to an indirect heat exchanger where it is increased from a temperature of 135 F. (corresponding to the heat content pick up in the first tower). The

water then going through the heat exchanger leaves at a final temperature of 213 F.

Analysis of the heat recovery in the above system shows an improvement corresponding to an increase in net steam output corresponding to 15.1%, which is equivalent to a reduction in fuel used, to achieve the same amount of steam production corresponding to 11.6%.

As another specific example of the operation of the two fluid heat transfer system employing the present organic medium chlorinated biphenyl having 48 wt. percent chlorine and with Water employed in a direct contact vessel with such medium, the following conditions indicate the amount of heat recovered in the system of FIG- URE 3. In this dual unit, the hot gases enter from duct 60 into the primary contacting tower 61 at the rate of 116,000 lbs. per hour at a temperature of 400 F. The tower is provided with liquid trap and withdrawal means 64 and 65. Upon passage through the first level of the tower, the exiting gases pass through the water spray tower 62, finally emerging at the top of the tower 63 at a temperature of 116 F. The synthetic organic medium from line 70 enters the lower portion 61 of the tower at 140 F. at the rate of 152,000 lbs. per hour, and leaves the tower at 300 F. by line 71 and pump 72. The heat acceptor fluid in this example is boiler feedwater which enters by line 80 into the top level 62 of the spray tower at a temperature of 60 F. and at the rate of 94,000 lbs.

This stream leaves the lower portion of the per hour. tower 62 having traps 6 5 by line 81 and pump 82 at a temperature of 135 F. An advantage of the dual contacting is that the water spray tower also functions to recover the organic medium which might otherwise be carried upwards from the first level. The further heating of the boiler ieedwater takes place in a direct contact heat exchanger 90 which is a vertical tank having one iiquid as the continuous phase and the other liquid disper-sed therein. For example, the synthetic organic medium flows downward countercunrent to the upward flow of the feedwater. The feedwater from line 83 enters this tower at a temperature of 135 F. having been increased in temperature as described above. After being further heated in the direct contact heat exchanger, the boiler feedwater leaves by line 94 and pump 95 at a temperature of 210 F. The synthetic organic medium from line 73 imparting heat to the water enters this heat exchanger at a temperature of 300 F. (the temperature at which this medium left the lower portion of the gas tower). The synthetic organic medium leaves the direct contact heat exchanger by line 92 and pump 93 at a temperature at 140 F. and at the rate of 152,000 lbs. per hour, and is then recycled.

In addition to the use of direct and indirect heat exchange, combinations of such systems are also a part of the present invention, for example by first passing the synthetic organic medium through a shell and tube heat exchanger, followed by a direct contact heat exchanger.

Analysis of the recovery in the above system shows an improvement corresponding to an increase of 15.5%

i said chlorinated polyphenyl compound by heat exchange from the said gas, and thereafter removing heat from the said chlorinated polyphenyl compound.

2. Process for the recovery of heat from hot gases which comprises passing the said hot gases into direct contact with a falling stream of droplets of chlorinated biphenyl having from 40% to 60% by weight of chlorine whereby [the temperature and heat content of the said chlorinated biphenyl is increased, and thereafter recovering the heat content of the said chlorinated biphenyl compound by passing the same in contact with a stream of an inert fluid.

3. Process for recovering heat from a stream of boiler flue gas which comprises passing the said gas countercurrent to a tailing stream of droplets of a chlorinated biphenyl compound having from 40 wt. percent to 60 wt. percent chlorine, increasing the temperature of the said chlorinated biphenyl compound by such contacting and thereafter recovering heat from the said chlorinated biphenyl compound.

4. Process for recovering heat from a stream of boiler flue gases which comprises first contacting the said rising stream of flue gas with a falling stream of droplets of a chlorinated biphenyl compound having trom 40 wt. percent to 60 wt. percent chlorine heating the said chloa .rina-ted biphenyl compound by absorbing heat from the said flue gases, passing the said heated chlorinated biphenyl compound to a heat exchanger, passing flue gases which have gone through the first contacting described above to a second contacting zone in which such flue gas is contacted with a falling stream of droplets of water, heating the said water by absorption of at least some of the heat of the said tflue gas stream, withdrawing the said heated water from the said contacting zone, and thereafter passing such heated water through the aforementioned heat exchanger and imparting additional heat to the said water from the chlorinated biphenyl compound therein.

References Cited by the Examiner UNITED STATES PATENTS 6/1958 Pil o et al. 2611-1151 X 6/196 2 Weingarten 25'266 X CHARLES SUKALO, ROBERT A. OLEARY,

Examiners.

M. A. ANT ONAKAS, Assistant Examiner. 

1. PROCESS FOR THE RECOVERY OF HEAT FROM HOT GASES WHICH COMPRISES PASSING THE SAID HOT GASES INTO DIRECT CONTACT WITH A STREAM COMPRISING A CHLORINATED BIPHENYL COMPOUND HAVING FROM 40% TO 60% BY WEIGHT OF CHLORINE, RAISING THE TEMPERATURE AND HEAT CONTENT OF THE 